A Matter of Impacts: Northwestern Geotechnical Engineering Research in Transportation

The links that form the nation’s transportation network – local streets, arterials, and highways; urban subways, commuter rail, and freight rail; airports and seaports, to name a few – often require heavy construction work in areas that are sensitive to disruption. Few transportation projects today are built in “greenfield” environments where there is no existing infrastructure. Instead, construction takes place in crowded areas where careful planning and monitoring are required to prevent disruption of nearby pipelines and utilities, unwanted soil movement, vibration complaints from occupants of nearby structures, and other side effects that might either stop the project entirely or increase the project cost dramatically through expensive mitigation efforts.

One of the most important challenges in geotechnics – the theory and practice of engineering for foundations, excavations, tunnels, roadbeds, earth dams, and other facilities in soil and rock, and the study of engineering properties of soil and rock themselves – is the highly variable nature of the geological deposits in and upon which engineers work. Modern manufacturing and quality control enable structural engineers to specify a particular grade of steel with confidence that the steel of a given grade will have uniform strength, ductility, and toughness. Geotechnical engineers, by contrast, must work with whatever materials were left on the site by natural processes. It is difficult and expensive to thoroughly characterize the engineering properties of the various deposits at a given major project site. This challenge gave rise to the observational method, a “learn-as-you-go” strategy in which observations and measurements during facility construction are used to refine the design as construction proceeds1. Successful application of the observational method depends upon thorough planning of how the design would change in light of various observations, selection of appropriate quantities to measure, and timely collection and analysis of measurements.

Research in geotechnics as it relates to transportation projects is one of the prominent legacies of Northwestern University’s Infrastructure Technology Institute (ITI). Through sponsorship and provision of technical assistance by the ITI Research Engineering Group (REG), ITI supports field research that has led to important new connections between theory, laboratory analysis, and performance of actual facilities.

ITI researcher Prof. Richard Finno and his team monitor the movement of soil around braced excavations in urban areas using a variety of instruments, including automated motorized total stations – essentially a robotic surveying instrument – networks of tilt sensors, and inclinometers. The monitoring data serve multiple purposes. First, they provide near-real time performance measurements for use in design updates, per the observational method. Second, the measurements provide a quantitative basis for protection of nearby facilities such as streets, subway tunnels, and pipelines from excessive soil movement. Third, through inverse analysis, comparison of expected and measured performance can be used to revise calculations of the stiffness of the soil deposit in which work is taking place; this updated characterization of the engineering properties of soil then facilitates more efficient designs for ongoing work in those deposits.

Some notable ITI-sponsored field investigations by Finno’s team include:

Monitoring and analysis of excavation performance during renovation of the Chicago Avenue station on the State Street subway in Chicago

Soil deformation monitoring during top-down construction of a mixed-use high rise building adjacent to two subway tubes and an in-service utility tunnel in downtown Chicago, including evaluation of a remotely operable in-place inclinometer

Deformation monitoring of soil containing high-pressure gas mains during excavation for a medical research center on Northwestern’s Chicago campus (pipelines fall under the jurisdiction of the US Department of Transportation)

Presently, Finno, his students, and the ITI Research Engineering Group are taking field measurements from two sites in the fast-growing South Loop area of Chicago. Monitoring of the basement slabs of the 53-story One Museum Park West condominium building was featured in ITI’s Winter 2010 newsletter2. In that case, the ITI REG and Finno’s students installed vibrating-wire strain gauges in the basement slab reinforcement just before the concrete was poured and monitored the strain in the slab as the concrete cured and the soil beneath the slab was mined out. Data from One Museum Park West are providing new insight into the performance of these slabs as excavation bracing during top-down construction. At Jones College Prep, a Chicago public high school about ten blocks from One Museum Park West, the ITI REG and Finno’s students installed an automated motorized total station to measure soil movement in three dimensions around an excavation for a new wing of the school immediately adjacent to South State Street and several 5-7 story buildings. The total station, and an experimental wireless network of tilt sensors on the adjacent buildings, is used to monitor the soil response – and the response of nearby infrastructure – as excavation proceeds. Through inverse analysis, the data will also be used to refine calculations of the engineering properties of the local soil deposits.

ITI researcher Prof. Charles Dowding has studied the effects of vibration from blasting, heavy construction, and industrial activity on buildings and underground infrastructure for over thirty-five years. Dowding’s work covers two particular challenges relating to ground vibration. The first is setting appropriate frequency-based limits for ground vibration from various sources upon different types of structures. While Chicago is a city built on clay, many cities – most notably New York City – are built in areas where rock is very near the surface. In these places, heavy construction often requires blasting to fragment the rock. For example, construction of the East Side Access tunnels, which will bring Long Island Railroad trains directly into Manhattan’s Grand Central Terminal, requires a considerable amount of blasting near columns supporting skyscrapers along Park Avenue. However, the guidelines for acceptable blast vibration levels near structures are based upon studies of one- and two-story wood-framed houses hundreds or thousands of feet from surface coal mine blasts. These two types of blasts are very different from close-in underground blasting in terms of dominant wave type and frequency content, and application of the surface coal mine blast regulations to close-in underground blasting results in over-conservative vibration limits, which in turn increase the cost and time needed to complete construction. Dowding is collaborating with partners from government and industry to develop more sensible frequency-based limits for blast vibration on urban mass transit construction projects.

The other challenge addressed by Dowding’s investigations of ground vibration effects on structures is more subtle, but no less important to the nation’s ability to build and sustain transportation infrastructure: ensuring the supply of construction aggregate by establishing data-driven guidelines for evaluating complaints of damage to structures near aggregate mines.

Some notable field investigations by Dowding’s team and the ITI REG include:

Crack response monitoring of the Blair House, the President’s guest house in Washington, DC, during reconstruction of Pennsylvania Avenue

Crack response monitoring of a house near an underground aggregate mine in Frankfort, Kentucky; ITI data were used by a federal judge to set ground vibration limits

Crack response monitoring of houses near road aggregate quarries in Franklin, Wisconsin; Naples, Florida; and Sycamore, Illinois; monitoring of various site allowed comparison of structural response to blasts within a few hundred feet of the house (Sycamore), a few thousand feet of the house (Franklin), and over a mile from the house (Naples).

Evaluation of two wireless sensor network technologies for structural response monitoring (one at Franklin, Wisconsin; the other at Sycamore, Illinois)

ITI is proud to support Northwestern research in geotechnical engineering – research with strong theoretical grounding, tempered by high quality field performance data – as these research efforts have a substantial past record and future promise to facilitate construction, rehabilitation, and renewal of transportation facilities in the nation’s increasingly dense urban environments.